Induction heating method for metal products

An induction heating method for metal products that continuously heats to a desired temperature a long metal product traveling at a constant speed through a plurality of induction heating coils disposed longitudinally and in tandem, comprising the steps of:

• controlling the frequency of the power supplied to the heating coils of the first-half groups so that the mean penetration depth of induced current in the metal product .delta. therefor equals (1/3 to 2/3)t (wherein t=wall thickness of the product);
• selecting and setting the frequency of the power supplied to the heating coils of the second-half group so that the ratio t/.delta..sub.T for the whole heating coils equals 1 (wherein t=mean wall thickness of the product, and .delta..sub.T =mean penetration depth for all heating coils of the second-half group);
• successively measuring the temperature of the metal product on the inlet side of the heating coil at longitudinal intervals thereof, each said interval being shorter than one-half of the heating coil length;
• calculating the longitudinally distributed temperatures of the metal product in the heating coil from the measured temperatures every time said temperature measurement is performed;
• determining the deviations between the calculated temperatures for the area closer to the outlet of the heating coil and the desired temperatures;
• and
• controlling the power supplied to the heating coil based on the determined deviations so that the product temperature on the outlet side of the heating coil equals the desired temperature.

An induction heating method for heating an elongated piece of metal moving continuously in a longitudinal direction, comprising the steps of:

• measuring the inlet temperature of the metal at longitudinal intervals with a non-contact thermometer located at the entry side of an induction heating coil;
• sectioning the induction heating coil into a plurality of successive sections in the longitudinal direction thereof, said sections equal in length to the longitudinal interval;
• detecting the speed of the metal by means of a speed detector;
• tracking the relative position between the metal and the induction heating coil by integrating the speed detected by the speed detector;
• measuring the voltage, current, and frequency of the electrical power applied to the induction heating coil;
• calculating the temperature of the metal in each respective section of the induction heating coil using the measured inlet temperature and speed of the metal and the measured voltage, current, and frequency of the electrical power applied to the induction heating coil;
• estimating the temperature of the metal at the exit side of the induction heating coil from the calculated temperatures and adjusting the electrical power applied to the induction heating coil to minimize the difference between the estimated metal temperature and the desired metal temperature at the exit side of the induction heating coil.